Spontaneous activity of otolith-related vestibular nuclear neurons in the decerebrate rat

Relationship between afterhyperpolarization profiles and the regularity of spontaneous firings in rat medial vestibular nucleus neurons

Our previous in vivo and in vitro whole-cell patch-clamp recording studies demonstrated that neurons in the medial vestibular nucleus (MVN) could be characterized on the basis of three electrophysiological properties: afterhyperpolarization (AHP) profile; firing pattern; and response pattern to hyperpolarizing current pulses. In the present study, to clarify which types of the classified MVN neurons correspond to neurons with regular or irregular firing, we investigated their spike discharge patterns using whole-cell patch-clamp recording in both in vivo and in vitro preparations. The discharge regularity was related to AHP profiles, and we found that: (i) the coefficient of variation (CV) of interspike intervals during spike discharges was smaller in neurons exhibiting AHP with a slow component [AHP(s+)] than in those without a slow component [AHP(s-)], or with a slow AHP component preceded by afterdepolarization (ADP) [AHP(s+) with ADP]; (ii) the blockade of Ca(2+)-dependent K(+) channels by 100 nm apamin abolished the slow component and increased the CV in neurons exhibiting AHP(s+); and (iii) the modulation of firing (firing gain) in response to ramp current was larger in neurons exhibiting AHP(s-) than in the other two neuronal types. These results suggest that neurons exhibiting AHP(s+) are regularly discharging neurons with small firing gains to stimulus, neurons exhibiting AHP(s+) with ADP are irregularly discharging neurons with small firing gains, and neurons exhibiting AHP(s-) are irregularly discharging neurons with large firing gains. The regular firing of neurons exhibiting AHP(s+) is attributed to the activation of apamin-sensitive Ca(2+)-dependent K(+) channels.

Spatial coding capacity of central otolith neurons

This review focuses on recent approaches to unravel the capacity of otolith-related brainstem neurons for coding head orientations. In the first section, the spatiotemporal features of central vestibular neurons in response to natural otolithic stimulation are reviewed. Experiments that reveal convergent inputs from bilateral vestibular end organs bear important implications on the processing of spatiotemporal signals and integration of head orientational signals within central otolith neurons. Another section covers the maturation profile of central otolith neurons in the recognition of spatial information. Postnatal changes in the distribution pattern of neuronal subpopulations that subserve the horizontal and vertical otolith systems are highlighted. Lastly, the expression pattern of glutamate receptor subunits and neurotrophin receptors in otolith-related neurons within the vestibular nuclear complex are reviewed in relation to the potential roles of these receptors in the development of vestibular function.

Expression of Trk receptors in otolith-related neurons in the vestibular nucleus of rats

The expression of the three Trk receptors (TrkA, TrkB, and TrkC) in otolith-related neurons within the vestibular nuclei of adult Sprague-Dawley rats was examined immunohistochemically. Conscious animals were subjected to sinusoidal linear acceleration along either the anterior-posterior (AP) or interaural (IA) axis on the horizontal plane. Neuronal activation was defined by Fos expression in cell nuclei. Control animals, viz labyrinthectomized rats subjected to stimulation and normal rats that remained stationary, showed only a few sporadically scattered Fos-labeled neurons. Among experimental rats, the number of Fos-labeled neurons and their distribution pattern in each vestibular subnucleus in animals stimulated along the antero-posterior axis were similar to those along the interaural axis. No apparent topography was observed among neurons activated along these two directions. Only about one-third of the Trk-immunoreactive neurons in the vestibular nucleus expressed Fos. Double-labeled Fos/TrkA, Fos/TrkB and Fos/TrkC neurons constituted 85-98% of the total number of Fos-labeled neurons in vestibular nuclear complex and its subgroups x and y. Our findings suggest that Trk receptors and their cognate neurotrophins in central otolith neurons may contribute to the modulation of gravity-related spatial information during horizontal head movements.

Fos expression in otolith-related brainstem neurons of postnatal rats following off-vertical axis rotation

To determine the critical time of responsiveness of developing otolith organ-related brainstem neurons and their distribution, Fos protein expression in response to off-vertical axis rotations (OVAR) was mapped in conscious Sprague Dawley rats from P5 to adulthood. OVAR was used to activate sequentially all utricular hair cells per 360 degrees revolution. We detected the coding of horizontal head positions in otolith organ-related neurons within the vestibular nucleus as early as P7. In the vestibular nuclear complex and its subgroups, the density of Fos-immunoreactive (Fos-ir) neurons increased steadily with age and reached the adult level by P21. In both labyrinthectomized rats subjected to OVAR and normal rats kept stationary, labeled neurons were found sporadically in the aforementioned brain regions in each age group, confirming that Fos labeling observed in neurons of normal experimental rats subjected to OVAR was due to otolith organ stimulation. Whereas OVAR-induced Fos-ir neurons were also first observed in vestibular-related brain areas, such as the prepositus hypoglossal nucleus, gigantocellular reticular nucleus, and locus coeruleus, of normal experimental rats at P7, those in the inferior olive were observed only from P14 onward. This indicates the unique maturation time of inferior olivary neurons in gravity-related spatial coding. In general, age-dependent increase in OVAR-induced Fos-ir neurons was observed in brain areas that received otolith inputs. The locus coeruleus was exceptional in that prominent OVAR-induced Fos-ir neuronal number did not change with maturation, and this was well above the low but significant number of Fos-ir neurons in control preparations. Taken together, our results suggest that neuronal subpopulations within the developing network of the horizontal otolith system provide an anatomical basis for the postnatal development of otolith organ-related sensorimotor functions. J. Comp. Neurol. 470:282-296, 2004.

Spontaneous discharge and response characteristics of central otolith neurons of rats during postnatal development

To study the developmental profile of otolith-related vestibular nuclear neurons, their spontaneous activities and response dynamics were examined in decerebrate rats aged seven, 14, 21 and 84 (adult) days. Extracellular recordings were performed in the lateral and descending vestibular nucleus of animals held at the stationary position in the earth-horizontal or subjected to constant velocity off-vertical axis rotation, which selectively stimulates the otolith receptors. All neurons displayed sinusoidal position-dependent modulation in discharge rate, indicating their capability in coding spatial information during low-frequency head movement. Some neurons showed a full-cycle response to off-vertical axis rotation (non-clipped), while other neurons were silenced in discharge during parts of each rotary cycle (clipped). In seven-day-old rats, three-quarters of the responsive neurons sampled were clipped and the proportion progressively decreased to less than one-quarter in adult rats. In each age group, the clipped neurons discharged in approximately 60% of the stimulus cycle. Response gains of the neurons increased with age, reaching a plateau from 21 days of age for clipped neurons and 14 days for non-clipped neurons. The clipped neurons demonstrated higher response gains than the non-clipped neurons at or beyond 21 days of age. Spontaneous activities of the neurons at the stationary and earth-horizontal positions were analysed in relation to their response gains; a positive correlation was observed from 14 days of age onwards. Both types of neurons showed progressive increase in spontaneous activity as the rats matured, though the clipped neurons exhibited significantly lower resting rates than the non-clipped neurons at each of the age groups studied. Some neurons that responded to off-vertical axis rotation were not spontaneously active at the stationary position, but the proportion of these decreased significantly with age. The coefficient of variation of each age group showed a bimodal distribution, thereby allowing spontaneously active neurons to be assigned as regular or irregular. Though the vast majority of both the clipped and non-clipped neurons showed irregular discharge patterns at seven days of age, the overall population became more regular as the rats matured. Irregular neurons of young rats exhibited phase-stable and phase-shift responses, while those of older rats showed only the phase-stable response. This distinction was not observed amongst regular neurons over the ages studied. Our results reveal features of central otolith neurons that can be taken as signs of maturation during the first three postnatal weeks. These neuronal features provide the framework for the analysis of behaviours mediated by the otolith system during postnatal maturation.

Neuronal response sensitivity to bidirectional off-vertical axis rotations: a dimension of imbalance in the bilateral vestibular nuclei of cats after unilateral labyrinthectomy

In decerebrate cats after acute hemilabyrinthectomy, the response sensitivity of extracellularly recorded vestibular nuclear neurons on the lesioned and labyrinth-intact sides were examined quantitatively during constant velocity off-vertical axis rotations with an aim to elucidate the functional contribution of otolithic inputs to the ipsilateral and contralateral vestibular nuclei. The bidirectional response sensitivity, delta, was determined as the ratio of the gain during clockwise to that during counterclockwise rotations. A continuum of response sensitivity was identified: one-dimensional neurons showed symmetrically bidirectional response patterns, while two-dimensional neurons showed asymmetrically bidirectional patterns that in some cases approached unidirectional patterns with change in velocity. The proportion of two-dimensional neurons was significantly increased after acute hemilabyrinthectomy. Two-dimensional neurons that responded only to one direction of rotation in at least one of the velocities tested were described as unidirectional neurons. This unidirectional response pattern was observed in one-third of the entire neuronal population studied, but not in cats with both labyrinths intact, thus suggesting that such prominent broadly tuned responses are normally masked by converging otolithic inputs from the contralateral side. These neurons were found in higher proportion on the lesioned side than on the labyrinth-intact side. Among the 70% of unidirectional neurons that exhibited bidirectional response at some velocities and unidirectional response at others, prominent shifts in delta values (i.e. between 0/infinity and finite values) with velocity can be computed for each neuron. The shifts in delta values correlated with large shifts in the response dynamics and spatial orientation as the response pattern changed with velocity. The response orientations of the unidirectional neurons pointed in all directions on the horizontal plane. When all the two-dimensional neurons (i.e. both the unidirectionally and bidirectionally responsive ones) were pooled, imbalances in the distribution of the response orientations and in response gain were found between the ipsilateral-side-down/head-down half-circle and the contralateral-side-down/head-up half-circle on the labyrinth-intact side, but not on the lesioned side. These results, derived from spatiotemporal processing of gravitational signals, reveal a novel dimension of imbalance between neuronal populations in the two vestibular nuclear complexes after acute lesion of one labyrinth. This feature would provide, on the one hand, deranged cues of spatial orientation and direction during slow head excursions and, on the other, a framework for the dynamic behavioral deficits associated with hemilabyrinthectomy.